Degumming of Antheraea yamamai silkworm cocoon

Abstract

Oak silkworm, Antheraea yamamai (A. yamamai), has been used for clothing and surgical suture and considered as biomaterial due to RGD tripeptide. This paper reported the degumming conditions of A. yamamai using sodium oleate, high pressure and temperature, and sodium carbonate. Degumming ratio of A. yamamai cocoon using sodium oleate was less than 10%. High pressure and temperature treatment induced 30% weight loss of A. yamamai cocoon. The concentration, treatment temperature and time using sodium carbonate was examined and revealed the following conditions for degumming; 5% owf, 60 min at 100℃. The degummed solution was confirmed using UV and FT-IR spectrometer. Our results can be used to handle A. yamamai silkworm cocoon for further application including material processing.

Introduction

Oak silkworms, Antheraea yamamai (A. yamamai) silkworm and Antheraea pernyi silkworm, are distributed on the far-east area including Korea, Japan, and northeastern China. Among them, A. yamamai silkworm spins green colored cocoon. The cocoon has been used for clothing, bullet-proof vests for warriors, bowstrings, fishing cords, musical strings, etc. Nowadays, A. yamamai silk has been considered as a possible novel biomaterial because it contains a tripeptide motif called RGD (Arg-Gly-Asp) (Datta et al., 2001; Minoura et al., 1995; Ruoslahti & Pierschbacher, 1986). Biocomaptibiltiy (Wang et al., 2011), gelation (Liu & Zhang, 2014), degradation (You et al., 2014), and PEG modified film (Wei et al., 2014) using A. yamamai silk protein have been investigated. As a possible food resources, the hydrolyzed A. yamamai silk fibroin powder was also examined due to rich glycine, alanine, and serine contents and various bioactivity (Kweon et al., 2006).

Cocoon material is composed of two kinds of proteins, a core protein (fibroin) and a gum-like protein (sericin). The content of sericin in the cocoon is different among the silkworm cocoons spun by silkworm kinds. To use A. yamamai silk fibroin for various application including clothing and biomaterial, degumming, a process to remove sericin, is an essential process. Reported degumming method for A. yamamai raw silk was sodium carbonate method and enzymatic method (Lee et al., 2013; Shin et al., 2012; Kawahara and Nakajima, 1992).

In this study, we examined the condition of A. yamamai cocoon degumming according to the temperature and concentration of sodium carbonate and measured the UV absorption characteristics of degummed solution and IR absorption characteristics of degummed A. yamamai cocoon.

 

Materials and Methods

Materials

A. yamamai silkworm cocoon was harvested at the farm of National Academy of Agricultural Science. Sodium carbonate, sodium oleate were purchased and used without further purification.

Degumming

The cocoon was degummed with physical method using autoclave and chemical methoud using various concentration of sodium salt and then washed with distilled water. Degumming loss percentage was calculated according to the following equation.

Degumming loss percentage (%) = (Wi – Wf) / Wi × 100

Where, Wi is initial weight of dry cocoon; Wf final weight of dry cocoon.

Structural characterization

Ultra violet spectrometer (Lambda 10, Perkin Elmer, USA) was used to measure the UV absorbance patterns of degummed Antheraea yamamai silkworm cocoon solution.

Fourier transform infrared (FT-IR) absorbance spectra were obtained using FT-IR spectrometer (Spectrum 100, Perkin Elmer, USA) in the spectral region of 1800 ~ 1000 cm-1 at a resolution of 2 cm-1 and 32 repeated scans were averaged for each spectrum.

 

Results and Discussion

Degumming condition of A. yamamai silkworm cocoon

To impose unique silk fabrics characteristics, raw silk has been degummed during the textile processing. Soap degumming is one of a common method for domestic silk degumming. The main component of soap is sodium oleate. Fig. 1 showed degumming ratio of A. yamamai and B. mori silkworm cocoon at 100℃. After 30 min treatment, domestic silkworm cocoon showed around 25%. On the other hand, even though 120 min treated, wild silkworm cocoon showed less than 10%.

Fig. 1.Degumming ratio of Antheraea yamamai silkworm cocoon and Bombyx mori silkworm cocoon with sodium oleate 12% owf aqueous solution at 100℃.

Wild silkworm A. yamamai cocoon has calcium oxalate crystal on its surface (Lee et al., 2015; Freddi et al., 1994). Calcium oxalate might prevent the action of sodium oleate to remove silk sericin from the filament. This data showed that soap degumming method is not good for A. yamamai silkworm cocoon.

Fig. 2 showed degumming ratio of A. yamamai and B. mori silkworm cocoon using autoclave. Sericin can be extracted by boiling the silkworm cocoon in water because sericin is water soluble protein (Kwak et al., 2013; Oh et al., 2011). B. mori silkworm cocoon was degummed about 20% with high pressure and temperature water. On the other hand, degummed ratio of wild silkworm cocoon showed about 30%.

Fig. 2.Degumming ratio of Antheraea yamamai silkworm cocoon and Bombyx mori silkworm cocoon with high pressure and high temperature at 120℃.

Fig. 3 showed degumming ratio of A. yamamai silkworm cocoon using sodium carbonate. Degumming ratio of A. yamamai cocoon was increased with degumming time and temperature. When A. yamamai cocoon were treated for 60 min at 100℃, the degumming ratio of the wild cocoon treated with 1% owf, 5% owf, 10% owf was 22.5%, 30%, and 28.2%, respectively. So, the best condition of alkali degumming for A. yamamai cocoon is 5% owf sodium carboante with 60 min at 100℃.

Fig. 3.Degumming ratio of Antheraea yamamai silkworm cocoon with sodium carbonate; sodium carabonate 1% owf aqueous solution (a), sodium carbonate 5% owf solution (b), sodium carbonate 10% owf solution (c).

Degumming ratio was reported various researchers using different degumming process. Shin et al. (2012) reported alkaline degumming ratio of A. pernyi and B. mori silkworm cocoon were 17.1% and 25.0%, respectively. Alkali degumming ratio of A. pernyi raw silk was 16.8% (Lee et al., 2013). Enzymatic degumming loss of raw silk spun by A. pernyi, A. yamamai, and A. assama silkworm were 11.6%, 16.0%, and 7.4% (Kawahara and Nakajima, 1992). Common Korean B. mori silkworm variety, Baegokjam, was reported 25.4% (Kweon et al., 2012).

UV spectra of A. yamamai sericin

A. yamamai degummed solution is expected to contain A. yamamai sericin extracted by degumming process. Sericin is a protein produced by silkworm. Generally, protein absorbs at 280 nm near-ultraviolet region due to the electron transfer of aromatic amino acid, tryptophan, tyrosine, and phenylalanine. As expected (Fig. 4), A. yamamai degummed solution showed strong absorption band at 280 nm. This phenomena is similar to the UV spectra of sericinjam sericin (Kim et al., 2009). The UV absorption intensity of degummed solution is coincidence with the degumming ratio of the wild silkworm (Fig. 3).

Fig. 4.UV spectra of Antheraea yamamai degummed solution

Fig. 5.FT IR spectra of degummed Antheraea yamamai cocoon.

FTIR spectra of degummed A. yamamai cocoon

FT-IR is a powerful technique to examine the conformation of protein. In particular, the position and intensity of amide bands are sensitive to molecular conformation of silk protein. The spectrum of wild silkworm cocoon showed strong absorption band at 1650 cm-1 (amide I), 1530 cm-1 (amide II), attributed to the α-helix conformation (Kweon et al., 2001). Amide I and amide II absorption bands were shifted to lower wavenumber due to the degumming process.

The degumming condition for A. yamamai silkworm cocoon was examined. The highest degumming condition is 5% owf sodium carbonate with 60 min at 100℃. The degummed solution showed strong UV absorption band at 280 nm and degummed cocoon showed amide band shift to lower wavenumber.